The present invention relates to a member for depositing a charge on a desired object in contact therewith, and a copier, facsimile apparatus, printer or similar image forming apparatus using it.
In the imaging art, there has been proposed a system wherein a charge depositing member applied with a voltage is held in contact with an image carrier or an image transfer member in order to charge the image carrier or to transfer a toner image from the image carrier to a transfer medium. This kind of system is desirable from the environment and energy saving standpoint and taught in, for example, "Fundamentals and Applications of Electrophotographic Technologies", the Institute of Electrophotographic Engineers of Japan, pp. 217-218 and pp. 318-302 (published by Corona), and Japanese Patent Laid-Open Publication Nos. 3-100579 and 3-202885. Japanese Patent Laid-Open Publication No. 2-311868, for example, discloses a charge depositing member implemented as a charge roller. The charge roller has a conductive core or shaft, an intermediate surface layer formed on the shaft and made of EPDM whose volume resistivity is 10.sup.4 .OMEGA.cm to 10.sup.5 .OMEGA.cm, a hydrin rubber layer formed on the intermediate surface layer, and a layer coating the hydrin rubber layer and made of nylon whose volume resistivity is 10.sup.8 .OMEGA.cm to 10.sup.11 .OMEGA.cm. The charging or image forming system using such a charge depositing member is advantageous over a system relying on discharge in that it produces a minimum of ozone and saves power.
The charge depositing member, or donor, of the kind described is usable not only to effect charging and image transfer in an image forming apparatus but also to deposit a charge on a desired object, or acceptor, in other various fields. The advantages over the system using a discharge are also available in respect of environment and energy saving.
The material forming the surface portion of the donor is required to have particular conductivity satisfying a required charge deposition characteristic. Such conductivity of the material may be provided by polar groups originally contained in the material or ions added to the material. This type of material will be referred to as a material of type A hereinafter, as distinguished from a material of type B, which will be described. Examples of the material of type A are hydrin rubber, nitril rubber, urethane rubber or similar polar rubber, and EPDM and silicone rubber to which is added various kinds of metal ion salts, surface active agents or similar ion agents.
The material of type A, containing polar groups and ions, exhibits a current-to-voltage characteristic conforming to the Ohm's rule. However, the problem is that the material of type A is susceptible to the environment, particularly humidity, since the polar groups and ions adsorb water. As a result, the electric resistance of this material noticeably changes depending on the environment. For example, the electric resistance increases by about 2 order on the transition from normal humidity (about 60%RH) to low humidity (about 15%RH) or decreases by about 1 order to 2 order on the transition from normal humidity to high humidity (about 90%RH). Assume that the surface of the donor is made of such a material. Then, when the resistance changes due to a change in environment, a voltage or current noticeably changes based on the Ohms' rule with the result that the charge deposition characteristic of the donor changes. Consequently, in the case of charging or image transfer, the charge characteristic or the image transfer characteristic changes.
To deal with the material of type A stated above, the environment (temperature and humidity) may be sensed so as to control the voltage to the donor on the basis of the varying environment. Alternatively, the current may be sensed before each operation so as to change the voltage in matching relation to the current. However, the environment sensing scheme is not satisfactory since a sensor responsive to the environment is problematic in accuracy and feedback delay. Particularly, when the donor is applied to an image forming apparatus, attractive images are not achievable unless the voltage is fully corrected against the environment.
Further, the change in voltage or current attributable to the environment causes the load acting on a power source, assigned to the donor, to change over a broad range. The power source, therefore, must have a great capacity, i.e., a high allowable upper limit of output voltage or output current. This increases not only the cost but also the size of the power source and obstructs the miniaturization of the entire apparatus.
While materials sufficiently small in the change of electric resistance and exhibiting a current-to-voltage characteristic matching the Ohm's rule are in development, an ideal material has not been reported yet.